As the core actuator of hydraulic systems, hydraulic cylinders suffer performance degradation caused by leakage, which directly impairs system pressure stability and operating efficiency, and may even trigger equipment failures and safety hazards. Hydraulic cylinder leakage falls into two categories: external leakage and internal leakage. The purpose of leakage testing is to identify leakage existence, locate leakage points and assess leakage severity, so as to provide a basis for maintenance.

Pre-test Preparation

Sufficient preparations shall be completed prior to testing.

1. Cleaning: Wipe off oil stains on the hydraulic cylinder surface and remove surrounding debris. Ensure the test area is well ventilated and free of flammable and explosive materials.
2. Ambient temperature: Excessively high or low temperature changes the viscosity of hydraulic oil and compromises test accuracy. Therefore, tests shall be carried out under specified ambient temperature, generally (20±5)℃.
3. Hydraulic oil inspection: Verify the type, liquid level and cleanliness of test hydraulic oil to meet system requirements and avoid test deviations caused by contaminated or insufficient oil.
4. Safe operation: Implement safety measures before testing, including system depressurization, power cut-off and protective arrangement.

External Leakage Test (Most Intuitive)

External leakage refers to oil escaping outside the cylinder. Three typical leakage locations are listed below:

• Oil leakage at sealing interfaces between cylinder barrel and end caps (or guide bushings);
• Outward oil leakage at the moving fit between piston rod and guide bushing;
• Oil leakage caused by poor sealing of cylinder pipe joints.

External leakage symptoms are obvious and require no sophisticated detection technology.

Test Methods

• Visual inspection: Pressurize the system and check for oil seepage at piston rod, cylinder barrel, end caps and oil ports.
• Wiping recheck: Wipe suspected areas dry, operate the equipment and observe recurring oil seepage.
• Static pressure observation: Shut down equipment and maintain pressure for several minutes to check slow oil seepage.

Internal Leakage Test (Most Critical)

Internal leakage means fluid cross-flow inside the cylinder, invisible externally, yet undermines output force and movement speed. Internal leakage mainly occurs at static seals between piston rod and piston, and dynamic seals between cylinder barrel inner wall and piston. Three common test methods are available:

1. Preliminary Visual Inspection

Conduct visual examination first: check for oil traces on the outer cylinder wall, and inspect piston rod surface for scratches, pitting and other damage. During equipment operation, watch out for abnormal noise such as cavitation sound and metal knocking sound; these phenomena usually serve as early warning signs of internal leakage.

2. Pressure Decay Test Method

This method does not directly measure actual internal leakage flow, but indirectly judges whether internal leakage complies with standards by comparing measured pressure drop with specified threshold values.

In practice, use an oil pump or intensifier to pressurize the test cylinder to specified test pressure, then the electrical control system commands the pressurization system to unload. The hydraulic lock installed on the cylinder pipeline maintains pressure inside the tested cylinder. During pressure holding, record the actual pressure drop readings from the pressure gauge. Compare the measured value with the allowable pressure drop standard of the cylinder to indirectly determine whether internal leakage meets acceptance criteria.

3. Stroke Detection Method

Before testing, eliminate thermal expansion and contraction induced by temperature variations; the temperature fluctuation of working environment shall be controlled within ±5℃.

Install magnetostrictive displacement sensors with precision up to 0.01 mm at both ends of the cylinder. Drive the cylinder to perform 10 reciprocating cycles, then compare the difference between actual stroke and theoretical stroke.

If the actual stroke is shortened by 3 mm or more relative to the theoretical stroke, leakage fault can be confirmed. For instance, for a cylinder with a design stroke of 300 mm, if the measured average stroke is only 296.5 mm, it satisfies the leakage judgment criterion.

Generally speaking, slow movement and insufficient output force of the cylinder mostly indicate internal leakage; oil stains on exterior surfaces point to external leakage; failure to build pressure or rapid pressure drop is highly likely caused by internal leakage. The above hydraulic cylinder leakage test methods can provide references for daily maintenance, fault diagnosis and troubleshooting of hydraulic systems. In practical application, technicians shall select appropriate test methods according to on-site working conditions and equipment configuration, and strictly follow operating procedures to ensure safe and stable testing.